Generally, a thermostat is installed between an engine and a radiator, and it is automatically opened or closed in response to a change of a coolant temperature such that an amount of coolant flowing to the radiator is regulated, and thereby a temperature of the engine is maintained to be constant so that overheating or overcooling of the engine can be prevented. Therefore, the engine may operate at its highest performance level. In addition, the thermostat increases efficiency of heating an inner space of a vehicle, prevents consumption of coolant, and maintains the temperature of the coolant.
Generally, there are pellet-type thermostats and bellows-type thermostats. Because the pellet-type thermostat is affected by pressure less than the bellows-type thermostat, the pellet-type thermostat can control temperature more precisely than the bellows-type thermostat and is thus more widely used.
If the thermostat is stuck in an open state, control of the amount of coolant flowing between the engine and the radiator cannot be normally performed. Therefore, the warm-up period for the engine becomes longer when compared to a normal state, so exhaust emission characteristics become deteriorated. For this reason, on board diagnostics regulation No. 2 (OBD2) forces monitoring of whether the thermostat is in a stuck-open state if the coolant temperature and intake air temperature are higher than −7°, and illumination of an MIL (malfunction illumination lamp) when it is determined that the thermostat is in the stuck-open state.
For this operation, an ECU (engine control unit) monitors the coolant temperature. A coolant temperature is used as an initial value, and a coolant temperature rising gradient is calculated using an engine load in response to a detected amount of air. Then, the ECU calculates a modeled coolant temperature.
The modeled coolant temperature is then compared with the actual coolant temperature. If the actual coolant temperature is lower than the modeled coolant temperature in a specific instance (e.g., an instance when the actual coolant temperature is 85°), it is determined that the thermostat is in a stuck-open state and the MIL is illuminated.
However, even if the thermostat operates normally, the actual coolant temperature rises very slowly in a cold area because the ambient air temperature is low. In addition, in an RV (recreation vehicle), the actual coolant temperature also rises slowly because the passenger room of the RV is relatively large so that the amount of coolant that is sent to the heater core is much larger than that of a normal vehicle. In such cases, even when the thermostats operate normally, it may be erroneously determined that the thermostat is in a stuck-open state.
A problem with the prior art is illustrated in FIG. 3. If it is determined that the actual coolant temperature A is lower than the modeled coolant temperature B at a specific instance in which the engine is sufficiently warmed up after starting the engine, the MIL is illuminated at C to notify the driver of that the thermostat is in the stuck-open state. Thus, in the thermostat monitoring method according to the prior art as stated above, it can be erroneously determined that the normally operated thermostat is in the stuck-open state when a vehicle is driven in a very cold area or in a low load state.
In addition, if a heater blower is operated such that heat from coolant flowing to a heater core is taken into the inner space of the vehicle in order to improve heating efficiency, the temperature of the coolant rises slowly. Under this situation, if a vehicle is driven in a low load state, the temperature of the coolant may drop, so that the MIL can be illuminated.
If the prohibition condition is strengthened in order to solve such a problem, the above-stated problem can be solved. However, regulations for monitoring times in the OBD-2 cannot be met.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.